This invention relates to improvements in rotary cutting tool structures for both turning and milling operations.
Rotary tool machining, in principle, uses a tool insert of circular shape mounted on a shaft held in bearings, both radial and thrust, to facilitate rotation thereof about its central axis. The tool can be self-propelled by the cutting action or can be driven externally.
Representative rotary tool constructions are disclosed in U.S. Pat. No. 2,233,724--Bannister, et al.; U.S. Pat. No. 2,513,881--Low; U.S. Pat. No. 2,689,498--Johnson; U.S. Pat. No. 2,885,766--Ernst, et al. and U.S. Pat. No. 3,329,065--Vaughn.
As is well recognized in this prior art, rotary tool machining offers significant advantages in that, because the cutting element rotates relative to the work, any given point on its peripheral cutting edge engages the workpiece for only a short interval of time and then moves on. As a result, the tendency of the cutting edge to heat up is greatly reduced, the tool life is extended and the cutting function is retained for a longer period of time. Extending the tool life lengthens the cutting time available between cutting insert changes.
In spite of the fact that rotary tool machining was introduced over one hundred years ago, the systems produced and proposed to date all lack adequate rigidity during the cutting operation. This inadequate dynamic rigidity results in chatter of the rotary tool holder, rapid chipping of the tool edge and accelerates wear in the bearing system. In the extreme case it may be necessary to stop the machining. The limited commercial use evident since the introduction of such tools is believed, at least in part, to be due to lack of a rigid rotary tool system. The problem of chatter addressed by this invention is particularly evident in the machining of difficult-to-machine materials, such as titanium alloys, nickel-base superalloys (e.g. Inconel 718) and hardened alloy steels (e.g. AISI 4340), which cause shear localization in the chips and large amplitude of forced oscillation (resulting in chatter) in the cutting process.
A wide variety of tool materials may be utilized in the practice of this invention. Such tool materials would include cemented carbides such as straight WC and multi-carbides in a cobalt binder, TiC in a Ni-Mo binder and coated carbides and ceramic materials such as alumina, mixtures of alumina with TiO, ZrO.sub.2 and/or TiC, and mixtures of silicon nitride with TiC, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3.